In IC fabrication, devices such as transistors may be formed on a semiconductor wafer or substrate, which is typically made of silicon. MOSFET devices are widely used in numerous electronic apparatus, including automotive electronics, disk drives and power supplies. These MOSFET devices often function as switches and are used to connect a power supply to a load.
The current flowing between the source regions and the drain region of MOSFET devices has often been measured using several mechanisms. Examples of such conventional mechanisms include using a sensing field effect transistors (SENSFETs), using sense resistors, or by measuring the voltage drop across the “on resistance” (RDS(on)). In a SENSFET, the source current is separated into a main circuit conducting the main current and a parallel measuring circuit.
These known mechanisms and devices unfortunately have several disadvantages. SENSFETs are expensive to manufacture and are not isolated from the MOSFET device, thereby complicating the operation of the semiconductor device containing them. Sense resistors, while being accurate, are disadvantageous because of the accompanying power dissipation. Measuring the current by using the voltage drop across RDS(on) has a low accuracy because of the statistical distribution (20–30%) of RDS(on) that results because of the particular MOSFET used. As well, the voltage drop mechanism is not very accurate because the RDS(on) depends on the temperature and can vary widely (i.e., 40–50% over a 100 degrees Celsius temperature range).